(1) Field of the Invention
This invention relates to (i) a nitrile group-containing highly saturated copolymer rubber, (ii) a process for producing the nitrile group-containing highly saturated copolymer rubber, (iii) a vulcanizable rubber composition comprising the nitrile group-containing highly saturated copolymer rubber, (iv) an aqueous emulsion of the nitrile group-containing highly saturated copolymer rubber, and (v) an adhesive composition comprising the nitrile group-containing highly saturated copolymer rubber.
The nitrile group-containing highly saturated copolymer rubber of the invention is characterized as exhibiting good high-rate vulcanizability and a vulcanizate made therefrom has high mechanical strength and low compression set. The adhesive composition comprising an aqueous emulsion containing the nitrile group-containing highly saturated copolymer rubber is especially useful for bonding under vulcanizing conditions a highly saturated nitrile rubber to a fibrous material.
(2) Description of the Related Art
In recent years, a nitrile group-containing highly saturated copolymer rubber (a typical example of which is a hydrogenation product of an acrylonitrile-butadiene copolymer) has attracted much attention as a rubbery material having good heat resistance, oil resistance and weather resistance. Many nitrile group-containing highly saturated copolymer rubbers have heretofore been proposed. For example, a hydrogenated acrylonitrile-conjugated diene copolymer rubber which is a hydrogenated product of an acrylonitrile-conjugated diene copolymer and has good ozone resistance and gasoline resistance is described in Japanese Unexamined Patent publication No. 54-132647.
A nitrile group-containing highly saturated copolymer rubber has a low degree of unsaturation, and therefore, is difficult to vulcanize and a vulcanizate thereof has a poor mechanical strength and a large compression set.
Pot injection molding of an acrylonitrile-conjugated diene copolymer rubber, a high fluidity and a high cross-linking efficiency at a high-temperature and short-time vulcanization are generally required. However, in general, the higher the vulcanization temperature, the lower the degree of crosslinking efficiency. Therefore injection-molded articles have poor tensile stress and impact resilience as compared with compression molded articles (for example, see Journal of Society of Rubber Industry, Japan, vol. 59, No. 4, p. 214-215, 1986). This tendency is conspicuous in a hydrogenated acrylonitrile-conjugated diene copolymer rubber.
To impart a high-rate vulcanizability to a hydrogenated acrylonitrile-conjugated diene copolymer rubber, many proposals have been made, which include incorporation of vulcanization promotors such as a tellurium dithiocarbamate compound (Japanese Unexamined Patent Publication No. 6-9822), a thiuram promotor plus a partial ester salt of a di- and/or tricarboxylic acid (Japanese Unexamined Patent Publication No. 4-264145) and a thiuram promotor plus N-trichloromethylsulphenylbenzenesulfanilide (Japanese Unexamined Patent Publication NO. 2-248442). However, the high-rate vulcanizability attained by these proposals is not on a high level and in some cases physical properties of the vulcanizate are deteriorated.
Further, contamination of a mold is conspicuous in the injection molding of a hydrogenated acrylonitrile-conjugated diene copolymer rubber wherein vulcanization is carried out at a high temperature. That is, where the molding is repeated, staining materials are gradually deposited on the mold with the result of contamination of moldings and Spoiling of the surface appearance. Therefore the mold must be cleaned at regular intervals, but the cleaning is time-consuming and costly and leads to reduction in productivity.
To avoid the contamination of a mold, proposals have been made wherein talc, sodium thiosulfate, carbon wax or silicone oil is incorporated in the copolymer rubber. However, it is difficult to avoid the mold contamination to any appreciable extent where the vulcanization is effected at a high temperature and a high rate as in injection molding.
With regard to the application of a highly saturated nitrile rubber for an adhesive, the present inventors proposed an adhesive composition comprising an aqueous emulsion of a highly saturated nitrile rubber and a resorcinol-formaldehyde resin, which is used for bonding a fibrous material with an adherend rubber, i.e., a highly saturated nitrile rubber similar to the rubber in the adhesive composition (Japanese Unexamined Patent Publication No. 63-248879 and 3-167239). Both the nitrile rubber in the adhesive composition and the nitrile rubber as the adherend rubber are highly saturated, i.e., have a low unsaturation. Thus, it is difficult to effect the vulcanization at a high efficiency and attain a high bonding strength.